Disc Brakes

Disc brakes appeared as early as 1902 but it wasn't until Jaguar's stunning Le Mans victory with the C-type in '53 that the technology really gained momentum. Keith Howard reports

As the redoubtable Le Mans campaigner John Wyer was to complain, "I think it was quite unfair to Jaguar to say they won Le Mans in 1953 because they went there with disc brakes. They won because they had a hell of a good motor car". But La Sarthe in 1953 was unquestionably where the disc brake came of age. Although it may not single-handedly have won the race for Jaguar, neither was the Coventry marque certain to have triumphed without it.

As disc brakes were already in use on aircraft by this time, it is often supposed that here lies another technology race car designers filched from aviation. Development of the aircraft disc brake was certainly an impetus, but historically the boot is on the other foot. Frederick William Lanchester, that colossus of the early British motor industry, patented the first disc brake in 1902 — over a year before Wilbur and Orville Wright took to the air in Kitty Hawk and tentatively launched the era of powered flight. Although Dr Lanchester's design was similar in all key respects to today's disc brake, however, even he made little use of it.

From that early beginning the disc brake makes occasional fleeting appearances in other cars and motorcycles before finally establishing itself in the 1950s. In 1914, for instance, Herbert Frood, founder of the brake friction material manufacturer Ferodo, designed a disc-based auxiliary brake system for the AC Cyclecar. Then the Great War intervened and Frood's brake vanished. Girling and Lockheed, the big names in automotive brake manufacture, both designed and manufactured disc brake systems during the '30s, but their applications — the Scout armoured car and George Eyston's Thunderbolt LSR challenger — were too other-worldly for the concept to catch on.

Other examples exist of the automotive disc brake being toyed with down the decades, but when Jaguar took up the idea in the early 1950s it was still a leap in the dark — one that many others proved remarkably reluctant to follow. Mercedes even went so far as to develop its bizarre, albeit effective, air brake for the 300SLR rather than abandon its drums. In Fl, where the cars' lightness ameliorated their braking limitations, the inevitable was widely postponed until late in the decade.

Setting history to one side there was, in any case, only so much that the designer of disc brakes for cars could learn from disc brakes developed for aircraft. Critically, the duty cycles are quite different in either application. An aircraft is typically required to brake only once every few hours, and even then may rely on assistance from reverse thrust, an arrester wire or brake 'chute. A racing car, by contrast, typically has to decelerate hard many times a minute and, even allowing for the high aerodynamic drag at work in many modem formulae, the wheel brakes still do most of the stopping.

It's little surprise, then, that Jaguar's new brakes did not work reliably from the box. Manufactured by the Dunlop Rim and Wheel Company, based opposite the old Jaguar factory in Swallow Road, the discs themselves were of mild steel, chromium plated to improve wear resistance, and the calipers were multi-pot items with three cylinders acting on three separate brake pads per side at the front. The master cylinder was a Girling unit and a Plessey pump, driven from the propshaft, energised a simple hydraulic servo system to lower pedal forces to an acceptable level.

As Stirling Moss was to comment in 'My Cars, My Career', "[Disc brakes] didn't just happen upon the scene like the greatest thing since sliced bread. Today, all production cars use them and they cause hardly any problems, but that's because we had all the problems in 1952! We would run them at one circuit, like Reims, say, where there were long straights along which the brakes could cool quite comfortably, and they'd give no trouble. But if we ran them on a tighter circuit with less space between the corners, enormous heat would build up and vaporise the fluid as soon as we began braking hard. Next time you braked the pedal would hit the floor."

Pad knock-back was another problem, with similar consequences but a quite different cause. Axial run-out of the disc under hard cornering, caused by flexibility or play in the wheel bearings, hub or disc mounting, would push the caliper pistons back into their bores so that the next time the brake was applied the entire pedal travel was taken up reuniting pads and disc. Pumping the brake would restore normal function but wasted time. A more permanent solution was found in adding balance pipes across the calipers, linking the pistons on either side. Steel spacers between pads and caliper pistons helped alleviate heat soak.

The C-type's first official outing using disc brakes was the 1952 Mille Miglia with Moss driving, but at that stage in their development they were still boiling the brake fluid. Only a persistent, seven days a week test and development programme eventually ironed out the problems. By 1953 the racing C-type enjoyed a significant and reliable braking advantage over its rivals.

Better able to dissipate heat than drum brakes and freed of drums' undesirable mechanical self-servo action (which could affect lateral stability and make modulating the braking effort more difficult), the disc brake established its superiority at Le Mans that year. Able to stop in about half the distance of other cars at the end of the Mulsanne straight, the C-types did much of their overtaking under deceleration going into Mulsanne corner. When the Hantilton/Rolt car crossed the finishing line to win, its superior braking contributed to an average speed of 105.85mph — the first time that 100mph had been cracked, and by a significant margin.

Over the next four years Jaguar would win Le Mans three more times using Dunlop's disc brakes, this time fitted to the D-type, and the following year Vanwall would take Goodyear's equivalents to the top in F1. The demonstration of discs' superiority was complete and even die-hard drum brake supporters were at last forced to recognise that the writing on the wall was now a death knell.